Bump xstream from 1.4.17 to 1.4.18 in /server

Bumps [xstream](https://github.com/x-stream/xstream) from 1.4.17 to 1.4.18.
- [Release notes](https://github.com/x-stream/xstream/releases)
- [Commits](https://github.com/x-stream/xstream/commits)

---
updated-dependencies:
- dependency-name: com.thoughtworks.xstream:xstream
  dependency-type: direct:production
...

Signed-off-by: dependabot[bot] <support@github.com>

server/pom.xml

@@ -149,7 +149,7 @@
         <dependency>
             <groupId>com.thoughtworks.xstream</groupId>
             <artifactId>xstream</artifactId>
-            <version>1.4.17</version>
+            <version>1.4.18</version>
         </dependency>
         <dependency>
             <groupId>com.googlecode.concurrentlinkedhashmap</groupId>